The flow behavior of concentrated suspensions consisting of a Newtonian matrix (hydroxyl-terminated-polybutadiene, HTPB) mixed with two different sizes of aluminum powder (30% and above by volume) and two different sizes of glass beads (50% and above by volume) was investigated. The effects of the solid content, particle size, type of solid particle material, and the temperature on the shear stress and viscosity of the composite systems were studied. Slip at the wall was an important factor in suspensions with high filler content, large particle size, and measurements carried out at high temperatures requiring corrections for slip at the wall effects. Suspensions with filler contents of 50% and above by volume showed non-Newtonian behavior, whereas suspensions with smaller contents of filler were Newtonian. In most cases, the non-Newtonian suspensions were pseudoplastic at small shear rates, but dilatant at high shear rates. The dilatancy became evident only upon correction for slip at the wall.